1. Field of the Invention
The present invention relates to an apparatus for driving a flat panel display, and more particularly, to an apparatus for driving a flat panel display which controls the amount of electrons emitted from a cell by controlling the output current of a cathode.
2. Description of the Related Art
Presently, liquid crystal displays are widely used as flat panel displays. The liquid crystal display makes an image by intermitting the light beam from a light source by using a liquid crystal. A driving scheme for the liquid crystal display includes a passive matrix addressing method and an active matrix addressing method.
The passive matrix addressing method applies different voltages to the upper and lower plates of a glass substrate of the liquid crystal display, and inputs data to a pixel at the intersection. With this method, pixels adjacent to the addressed pixel are affected from the applied voltages, and thus a compensating circuit is necessary to implement a clear display. As a result, the implementation of a driving circuit becomes complex.
The active matrix addressing method has one cell transistor and one capacitance per pixel. According to the active matrix addressing method, one pixel continues to be driven by the previous pixel data until the next pixel data is inputted. This method is aimed at improving the image quality and of making the driving circuit simple.
With the active matrix addressing method, improvement of image quality and simplification of the driving circuit can be achieved. A large number of transistors and capacitors, however, must be implanted on the glass substrate of the liquid crystal display. Thus, this method has drawbacks in that the manufacturing process is very complex and the yield of the process is lowered.
This liquid crystal display occupies a large amount of market share. Since only a small percentage of light from the light source contributes to a display, power consumption is high. Also, it is difficult to make a large scale display. Since a material of the quasi-liquid state (liquid crystal) is used, the display is sensitive to change in atmospheric temperature. It is also vulnerable to pressure. The screen of the display is not very bright, the resolution of the display is very limited, and thus there are few applications.
The field emission display has been proposed as the new flat panel display to overcome the afore-mentioned problems. The field emission display displays a screen in a way similar to how a cathode ray tube makes use of emitted electrons. The field emission display differs from the cathode ray tube in that the former uses a cold electron emission.
The field emission display disposes field emission devices which emit electrons for each pixel, and makes a displayed image by making the electrons from the field emission devices collide with electrodes coated with a fluorescent film.
The field emission display has been widely accepted as the next generation flat panel display which can solve the above-mentioned problems where: the power consumption is large, it is difficult to make a large scale display, the display is sensitive to change in atmospheric temperature, the display is vulnerable to pressure, the screen of the display is not very bright, and the resolution of the display is very limited, all of the above resulting only in a few applications.
The field emission display can integrate several hundreds to several thousands of electron emission devices to form a pixel. The electron emission device, which forms a pixel of the field emission display, comprises of cathode 52 which is connected to base electrode 50, gate electrode 54 disposed apart from and above cathode 52, anode 58 with its back side coated with fluorescent film 56 as shown in FIG. 1. Fluorescent film 56 allows an image to be displayed by generating a light corresponding to the amount of electrons which collide with the fluorescent film.
Anode 58 attracts the electrons emitted from cathode 52. Anode 58 is transparent so that the light of fluorescent film 56 can be transmitted. Cathode 52 has a shape of a cone which forms the upper part of a tip. Cathode 52 makes electrons emitted from its tip by means of a driving source from base electrode 50.
Gate electrode 54 induces the emission of electrons from cathode 52 by a high voltage which is lower than the voltage applied to anode 58. The emitted electrons are directed to anode 58 to which the high voltage has been applied.
All flat panel displays including the field emission display which is equipped with field emission devices, adopt a pulse width modulation (PWM) driving scheme and a pulse amplitude modulation (PAM) driving scheme. The PWM driving scheme modulates the width of the output pulse and controls the gradation of a display. The PWM driving scheme requires a high speed operating frequency and transistors have to be integrated per cell. Thus the manufacturing process is complex and the cost associated with manufacturing is increased. The PAM driving scheme modulates the level of the output voltage or current, and has a drawback where a separate circuit for dividing a voltage or a current is required.
U.S. Pat. No. 5,103,145 discloses a display gradation scheme wherein one pixel is divided by a cathode of 1:2:4:8. This scheme also has a drawback where the brightness is lowered due to the division of the cathode with respect to a limited number of pixels, and that the amount of current cannot be compensated for against the difference between cathodes due to different cathode manufacturing processes.